libforum rebuilds every time install is run

[rhdtownsend]

When ./install is re-run after a complete build, libforum (in the forum subdir) gets rebuilt even though it doesn't need to be. Same with gyre. This will likely get fixed when I replace both modules with more-recent versions (I've recently revised the build systems for both). Just flagging this as something that needs attention.

[VincentVanlaer]

Fwiw, with #724 forum doesn't get rebuild anymore when you start the build process again.